Gray-scale photomask can be used for manufacturing 3D (three-dimensional) micro-nano structures, and particularly have large advantage in fabricating a micro-optical element. Furthermore, it also can be used for manufacturing micro-electro-mechanical systems. The three-dimensional processing with gray-scale photomask technology is achieved by changing the light transmittance at different locations of the mask so as to control the exposure intensity of the corresponding locations in photosensitive resist while exposed under light. The most outstanding gray-scale photomasks are ones based on halftone gray-scale mask and high-energy beam-sensitive (HEBS) glass, wherein the halftone gray-scale mask is manufactured with conventional photolithography method by changing the size or density of openings with a diameter on an order of sub-wavelength in Cr thin metal film such that the light transmittance of the opening can be changed in approximately continued gray-scale (Reimer K, Quenzer H J., SPIE, 1997, 3008:27902881). However, the main drawback of this method is its complex processing: it involves all steps of the conventional photolithography processing, and a high resolution photolithography is required for manufacturing the mask; furthermore, the related design in structure and the gray-scale calibration are also complicated. Another gray-scale material used widely is a HEBS glass produced by Canyon Material Inc., on which the high energy electron beam directly writing is performed, such that the unexposed portion becomes transparent, and transmittance of the exposed portion is reduced due to the silver particles created by the decomposition of the material, hence, the change in the transmittance is dependent on the exposure dose (U.S. Pat. No. 5,078,771, 1992). But it is very complicated to manufacture this material, because in the processing of this material, a zinc crown glass having a low expansion rate is used as a substrate, which is placed and heated in an acidic solution after a series of treatment, e.g. polishing etc, and after a long time of ion-exchange, a layer of complex crystal, which has a typical thickness of 3 micron is formed. Additionally, some light passivant is needed to dope with the crystal so as to widen the bandwidth of the energy band. As a result, the production cost is very high owing to the above-mentioned complicated process, and the price of gray-scale mask being made of HEBS glass further increases plus with the electron beam direct-writing under vacuum. Therefore, the high costs of the above two gray-scale masks make it difficult to apply in industry on a large scale.
Accordingly, it has very important practical value to develop a cost-effective gray-scale photomask.